1. Field of the Invention
This invention relates to a union nut suitable for drawing together and compressing axially two items to be joined together and, although it is not restricted to that field, the nut of the invention is particularly applicable to the art of coupling together portions of tubing, and for ease of illustration of the nature and advantages of the invention a preferred embodiment will be described later herein by way of non-limiting example.
2. Description of the Prior Art
In the connection of portions of tubing placed end to end, it is conventional to place a resiliently compressible seal element between axially-facing and faces of the respective portions, and to exert exial pressure on the two portions to compress the seal of good fluid-tight engagement. In the commercial use for such joints, and especially for example in the brewing and chemical industries, the joint may be subjected to quite high changes and temperature, and this has been known to cause jamming of a threaded portion of a union nut on a threaded end portion of one of the tubes, due especially to differences in the coefficient of expansion of the nut material relative to the tube material, and to the possibility of corrosion occurring between the threading of the nut and the threading of the tube. Such jamming may be avoided or at least reduced by making the nut of a material having a similar or equal coefficient of expansion as the tube part, and/or making the nut of a material having very low surface-to-surface friction and less liability to corrode, e.g. of plastics material. However, the nut must also be subjected to high forces for tightening and loosening of the joint, e.g. by the application of a tool such as a spanner, and it is found that plastics materials used in commercially-practicable thicknesses do not stand up to the manipulation required.
In a conventional type of end-to-end joint for two portions of tube, the end of one tube is provided with an external threading, and the end of the other tube portion is provided with a lip or flange which can be abutted by a flange of the union nut so that axial drawing-together of the two tube portions may be obtained by tightening the nut onto the threaded tube portion. Such tube portions may form part of a fixed installation in which it is undesirable to have to release or to cut the tube portions, but nevertheless the joint may need releasing and tightening from time to time, e.g. for cleaning and/or seal replacement. As the union nut must include a flange capable of abutting against the flange of the respective tube portion, the nut can only be engaged onto the tube portion by bringing it up to the tube flange from a remote position, i.e. the tube must be wholly released, or cut, to permit the nut to be placed about it. This is a considerable disadvantage in the use of such unions, and it is accordingly a second object of the invention to provide an improved union nut of which part can be engaged onto the flanged tube portion from the direction of the seal element and, after proper assembly, the nut can then be used in the usual way by abutting against the tube flange, thereby obviating the cutting or removal of that tube portion at each time of making up or releasing the joint. It is accordingly a first object of the invention to provide an improved union nut in which a portion which provides the threading action is made of a material which is inherent less likely to bind onto a threaded tube, and which nevertheless can stand up to the heavy forces required during tightening and loosening.